Input/output (“I/O”) circuits are used to input electronic signals to and output electronic signals from integrated circuits. A typical integrated circuit (“IC”) includes an integral I/O circuit for each of its externally accessible I/O pins. An I/O circuit usually includes a driver circuit which receives signals from the IC and outputs them to the I/O pin. It also generally includes an input buffer which receives signals from the I/O pin and inputs them to the IC. A typical I/O circuit also includes an enable circuit which can place the driver circuit in either a high impedance state in which signals can be input to the IC via the I/O pin, or in an output enabled state in which signals can be output from the IC via the I/O pin.
I/O circuits transfer signals to and from integrated circuit devices in a variety of types of electronic systems. For instance, I/O circuits may be used to interconnect integrated circuits to a shared system bus so that multiple ICs connected to the bus can communicate with each other. In many electronic systems all of the ICs connected to a system bus operate at the same supply voltage level. However, as the dimensions of the circuits in ICs have decreased, the supply voltages employed by ICs also have decreased. As a result, there has been a proliferation of mixed signal systems in which some ICs connected to a system bus operate at a higher supply voltage (e.g., 3.3-volts), and other ICs connected to the same system bus operate at a lower supply voltage (e.g., 1.65-volts).
A voltage regulator may be used to enable circuits/systems to operate using only one supply voltage from a power supply, with the voltage regulator providing various subcircuits and/or subsystems with different individual supply voltages. However, timing dead zone problems, which may cause hot carrier injection and gate oxide integrity issues, and power sequence problems have been encountered with multiple power domains. Thus, improved methods, systems, and apparatus for regulating power supplies are desirable.